The present invention relates to a thin film device transfer method, a thin film device, a thin film integrated circuit device, an active matrix board, a liquid crystal display, and an electronic apparatus.
In the manufacture of a liquid crystal display comprising thin film transistors (TFTs), for instance, thin film transistors are formed on a substrate through CVD or any other process. Since a fabrication process for forming thin film transistors on a substrate involves high-temperature treatment, a substrate made of a heat-resistant material is required, i.e., the substrate material must have a high softening point and a high melting point. At present, therefore, quartz glass is used as a material to provide a substrate capable of withstanding up to approx. 1000xc2x0 C., or heat-resistant glass is used as a material to provide a substrate capable of withstanding up to approx. 500xc2x0 C.
As mentioned above, the substrate on which thin film devices are to be mounted must satisfy conditions required for fabricating the thin film devices. Namely, the kind of substrate is determined to meet fabrication conditions required for the devices to be mounted thereon.
In view of a subsequent phase to be taken after thin film devices such as TFTs are formed, the substrate indicated above are not always preferable.
Where a fabrication process involving a high-temperature treatment is carried out, a quartz glass or heat-resistant glass substrate is used as exemplified above. However, the quartz glass or heat-resistant glass substrate is very expensive, resulting in an increase in product cost.
The glass substrate is also disadvantageous in that it is relatively heavy and fragile. A liquid crystal display for use in a portable electronic apparatus such as a palm-top computer or mobile telephone should be as inexpensive as possible, light in weight, resistant to deformation to a certain extent, and invulnerable to dropping. In actuality, however, the glass substrate is heavy, not resistant to deformation, and vulnerable to dropping.
In other words, there is a discrepancy between restrictive conditions required for a manufacturing process and characteristics desirable for a manufactured product. It has been extremely difficult to satisfy both of these required process conditions and desirable product characteristics.
The inventors, et al. have proposed a technique in which an object-of-transfer layer containing thin film devices is formed on a substrate through a conventional process and thereafter the object-of-transfer layer containing thin film devices is removed from the substrate for transference to a destination-of-transfer part (Japanese Patent Application No. 225643/1996). In this technique, a separation layer is formed between the substrate and a thin film device which is the object-of-transfer layer, and the separation layer is irradiated with light to cause exfoliation in an inner-layer part and/or interface of the separation layer. Thus, bonding strength between the substrate and the object-of-transfer layer is weakened to enable removal of the object-of-transfer layer from the substrate. In this manner, the object-of-transfer layer is transferred to the destination-or-transfer part. Where a fabrication process for forming thin film devices involves high-temperature treatment, a quartz glass or heat-resisting glass substrate is used. In the technique mentioned above, however, since the destination-of-transfer part is not exposed to high-temperature treatment, restrictive requirements imposed on the destination-of-transfer part are advantageously alleviated to a significant extent.
When the object-of-transfer layer containing thin film devices is removed from the substrate employed for thin film device formation so that the object-of-transfer layer is transferred to the destination-of-transfer part, the layering relationship of the object-of-transfer layer with respect to the destination-of-transfer part becomes opposite to that of the object-of-transfer layer with respect to the substrate. Namely, the side of the object-of-transfer layer which has faced the substrate originally does not face the destination-of-transfer part. For example, in the case where the object-of-transfer layer has first and second layers and is formed on the substrate in the order of the first and second layers, when the object-of-transfer layer is transferred to the destination-of-transfer part, the object-of-transfer layer is configured thereon in the order of the second and first sub-layers.
In the common practice of forming thin film devices on a substrate, electrodes are formed via an insulation layer after the formation of the element. Since the electrodes are disposed on the surface side, wiring connections or contacts can be arranged on the electrodes with ease. On the contrary, where the object-of-transfer layer containing thin film device and electrodes is transferred to the destination-of-transfer part, the electrodes are covered with the destination-of-transfer part, making it difficult to arrange wiring connections or contacts thereon.
In view of the foregoing, it is a general object of the present invention to provide a novel technique in which a substrate employed for thin film device formation and a substrate used as an actual element of a product (i.e., a substrate having characteristics desirable for usage of the product) can be selected individually and flexibly and in which thin film devices can be transferred to the substrate used as the actual product element while maintaining the layering relationship of the thin film devices with respect to the substrate employed for thin film device formation.
In accomplishing this object of the present invention and according to one aspect thereof, there is provided a thin film device transfer method comprising:
a first step of forming a first separation layer on a substrate;
a second step of forming an object-of-transfer layer containing a thin film device on the first separation layer;
a third step of forming a second separation layer on the object-of-transfer layer;
a fourth step of attaching a primary destination-of-transfer part to the second separation layer;
a fifth step of removing the substrate from the object-of-transfer layer using the first separation layer as a boundary;
a sixth step of attaching a secondary destination-of-transfer part to the bottom of the object-of-transfer layer; and
a seventh step of removing the primary destination-of-transfer part from the object-of-transfer layer using the second separation layer as a boundary,
whereby the object-of-transfer layer containing the thin film device is transferred to the secondary destination-of-transfer part.
The first separation layer to be separated later is provided on a substrate such as a quartz glass substrate having high reliability for device fabrication, and the object-of-transfer layer containing thin film devices such am TFTs is formed thereon. Then, the second separation layer to be separated later is formed on the object-of-transfer layer, and further the primary destination-of-transfer part is attached to the second separation layer. Thereafter, using the first separation layer as a boundary, the substrate employed for thin film device formation is removed from the object-of-transfer layer. In this state, however, layering relationship of the object-of-transfer layer with respect to the primary destination-of-transfer part is opposite to that of the object-of-transfer layer with respect to the substrate employed for thin film device formation.
It is therefore preferable that the first separation layer is removed from the bottom of the object-of-transfer layer, then the secondary destination-of-transfer part is attached to the bottom thereof. Thereafter, using the second separation layer as a boundary, the primary destination-of-transfer part is removed from the object-of-transfer layer. Thus, the secondary destination-of-transfer part is disposed at the position that has been occupied by the substrate employed for thin film device formation, i.e., the layering relationship of the object-of-transfer layer with respect to the secondary destination-of-transfer part agrees with that of the object-of-transfer layer with respect to the substrate employed for thin film device formation.
The step of attaching the secondary destination-of-transfer part to the bottom of the object-of-transfer layer and the step of removing the primary destination-of-transfer part from the object-of-transfer layer may be reversed; namely, either of these steps may be carried out first. In a situation where a problem may occur in handling the object-of-transfer layer after removing the primary destination-of-transfer part, however, it is desirable to attach the object-of-transfer layer to the secondary destination-of-transfer part first and then remove the primary destination-of-transfer part from the object-of-transfer layer. In this respect, the primary destination-of-transfer part may be made of any material having at least a shape-retaining property. Since the primary destination-of-transfer part is not used in thin film device formation, it is not required to consider restrictive process conditions such as heat resistance and metallic contamination.
In the fifth step, it is preferable to include a step of irradiating the first separation layer with light to cause exfoliation in an inner-layer and/or interface of the first separation layer.
The first separation layer is irradiated with light, thereby causing a phenomenon of exfoliation in the first separation layer. This decreases adhesiveness between the first separation layer and the substrate. Then, by applying force to the substrate, the substrate can be removed from the object-of-transfer layer.
The substrate preferably transmits light. In this case, the first separation layer is irradiated with light through the light transmitting substrate.
Thus, without directly irradiating the thin film devices with light, exfoliation can be made to occur in the first separation layer, thereby reducing the possibility of degradation in the performance characteristics of the thin film devices.
Further, the second separation layer may be made of an adhesive material. In this case, the fifth step includes a step of melting the adhesive material.
Where the second separation layer made of an adhesive material is employed, it is also usable as an adhesive for attaching the primary destination-of-transfer part later. Still more, after the primary destination-of-transfer part is attached, it can be removed easily by heating. Still further, even in case there is some unevenness on the surface of the object-of-transfer layer containing thin film devices, the adhesive material can be used as a flattening layer to compensate for the unevenness, thus making it easy to attach the primary destination-of-transfer part to the second separation layer.
In the seventh step, it is preferable to include a step of irradiating the second separation layer with light to cause exfoliation in an inner-layer part and/or interface of the second separation layer.
The second separation layer is irradiated with light, thereby causing a phenomenon of exfoliation in the second separation layer. This decreases adhesiveness between the second separation layer and the primary destination-of-transfer part. Then, by applying force, the primary destination-of-transfer part can be removed from the object-of-transfer layer.
The primary destination-of-transfer part preferably transmit light. In this case, the second separation layer is irradiated with light through the light transmitting primary destination-of-transfer part.
Thus, without directly irradiating the thin film devices, exfoliation can be made to occur in the second separation layer, thereby reducing the possibility of degradation in the performance characteristics of the thin film devices.
In the second step, it is preferable to include a step of forming an electrode for conduction to the thin film device after formation of the thin film device. In this case, the secondary destination-of-transfer part, thin film devices, and electrodes are superimposed in the order mentioned. Even after the object-of-transfer layer is transferred to the secondary destination-of-transfer part, wiring connections or contacts can be arranged on the electrodes with ease.
Further, it is preferable to provide a step of removing the second separation layer from the object-of-transfer layer. In this step, the second separation layer, which is unnecessary, is removed completely.
Referring particularly to preferable properties of a material of the secondary destination-of-transfer part, the secondary destination-or-transfer part is not used in thin film device formation as in the case of the primary destination-of-transfer part. Therefore, in selection of a material for the secondary destination-of-transfer part, it is not required to consider restrictive process conditions such as heat resistance and metallic contamination.
The secondary destination-of-transfer part may be a transparent substrate. An inexpensive soda glass substrate or a flexible transparent plastic film may be considered as the transparent substrate. Where a transparent substrate is used as the secondary destination-of-transfer part, it is possible to realize a liquid crystal panel substrate having thin film devices formed thereon, for example.
Assuming that a maximum temperature in the formation of the object-of-transfer layer is Tmax, the secondary destination-of-transfer part is preferably made of a material having a glass transition temperature (Tg) or softening point lower than or equal to the Tmax.
Thus, it becomes possible to flexibly use an inexpensive glass substrate which has not been applicable conventionally because of insufficient resistance to the maximum temperature in device formation. Similarly, the primary destination-of-transfer part is not required to have resistance to heat as high as the maximum temperature level in the process of thin film device formation.
The glass transition temperature (Tg) or softening point of the secondary destination-of-transfer part may be lower than or equal to the maximum temperature in the process of thin film device formation since the secondary destination-of-transfer part is not exposed to the maximum temperature of thin film device formation. The secondary destination-of-transfer part may therefore be made of a synthetic resin or glass material.
For instance, if a flexible synthetic resin sheet such as a plastic film is used as the secondary destination-of-transfer part and the thin film devices are transferred thereto, it is possible to provide an advantageous characteristic which would not be attained with a highly rigid glass substrate. By applying the present invention to liquid crystal display manufacturing, a display device which is flexible, light-weight and invulnerable to dropping can be realized.
Further, for instance, an inexpensive soda glass substrate is also usable as the secondary destination-of-transfer parts. The inexpensive soda glass substrate is advantageous in lowering the manufacturing cost. Since the soda glass substrate gives rise to a problem that alkaline components thereof are eluted through heat treatment in TFT fabrication, it has been difficult conventionally to use the soda glass substrate in manufacturing an active matrix liquid crystal display. In the present invention, however, since the thin film devices already completed are transferred to the secondary destination-of-transfer part, the problem concerning heat treatment does not take place. It is therefore possible to use the soda glass substrate or the like having such a problem as mentioned above in a field of an active matrix liquid crystal display.
Then, referring particularly to properties of a material of the substrate on which the object-of-transfer layer is formed, it is preferable for the substrate to provide heat resistance. In thin film device formation, heat treatment at a desired temperature can thus be carried out to enable fabricating high-performance thin film devices with high reliability.
Furthermore, the substrate mentioned above preferably allows transmission of 10% or more of light to be used for exfoliation thereof. A sufficient amount of light energy for exfoliation in the first separation layer can thus be transmitted through the substrate.
Assuming that a maximum temperature in formation of the object-of-transfer layer is Tmax, the substrate is preferably made of a material having a distortion point higher than or equal to the Tmax.
Thus, it becomes possible to carry out heat treatment at a desired temperature in thin film device formation for fabricating high-performance thin film devices with high reliability.
Then, referring to preferable properties of a material of the first separation layer and/or second separation layer in which exfoliation is made to occur by irradiation with light, the first separation layer and/or second separation layer are preferably made of amorphous silicon.
Amorphous silicon absorbs light, and it is rather easy to manufacture and highly practicable to use.
Further, it is preferable to use amorphous silicon which contains 2 atomic % or more of hydrogen (H).
Where amorphous silicon containing hydrogen is used, irradiation with light causes release of hydrogen, thereby producing internal pressure in the separation layer. Thus, an action for promoting exfoliation in the separation layer takes place.
Amorphous silicon containing 10 atomic % or more of hydrogen (H) may also be used.
An increase in percentage content of hydrogen increases the degree of action for promoting exfoliation in the separation layer.
As another kind of material for the first separation layer and/or second separation layer in which exfoliation is made to occur by irradiation with light, silicon nitride may be used.
Further, as another kind of material for the first separation layer and/or second separation layer in which exfoliation is made to occur by irradiation with light, a hydrogen-containing alloy may be used.
Where a hydrogen-containing alloy is used as a material for the separation layer, irradiation with light causes release of hydrogen, thereby promoting exfoliation in the separation layer.
Still further, as another kind of material of the first separation layer and/or second separation layer in which exfoliation is made to occur by irradiation with light, nitrogen-contained alloy may be used.
Where a nitrogen-containing alloy is used as a material of the separation layer, irradiation with light causes release of nitrogen, thereby promoting exfoliation in the separation layer.
The separation layer may be a single-layer film or a multi-layer film which comprises an amorphous silicon layer and a metallic layer formed thereon.
As another kind of material of the first separation layer and/or second separation layer in which exfoliation is made to occur by irradiation with light, any material including at least one of the following components may be used; ceramic, metal, and high polymer.
These substances are indicated here as representative groups that are applicable in practice as the first separation layer and/or second separation layer in which exfoliation is made to occur by irradiation with light. For example, a hydrogen-containing alloy or nitrogen-containing alloy may be selected from the group of metallic materials. In the case where a hydrogen-containing alloy or nitrogen-containing alloy is used, exfoliation in the separation layer is promoted by release of hydrogen gas or nitrogen gas due to irradiation with light similar to amorphous silicon.
The light used in the irradiation step is preferably a laser beam.
A laser beam has a coherent property, and is suitable for causing exfoliation in the first separation layer and/or second separation layer.
It is possible to use a laser beam having a wavelength of 100 nm to 350 nm.
The use of laser energy light having a short wavelength enables efficient exfoliation in the first separation layer and/or second separation layer.
As an example of a laser beam meeting the requirement mentioned above, excimer laser is preferable. An excimer laser is a kind of gas laser which is capable of outputting a high-energy laser beam having a short wavelength in the ultraviolet region. A combination of rare gas (Ar, Kr, Xe) and halogen gas (F2, HCl) is used as a laser medium to produce laser beam energy at each of four wavelengths (XeF=351 nm, XeCl=308 nm, Krf=248 nm, ArF=193 nm).
In the first separation layer and/or second separation layer, irradiation with an excimer laser causes direct disconnection of molecular bonding, gaseous evaporation, etc. without a thermal effect.
A laser wavelength ranging from 350 nm to 1200 nm may be adopted.
For inducing separation by such a phase change action as release of gas, vaporization or sublimation in the first separation layer and/or second separation layer, a laser beam having a wavelength of 350 nm to 1200 nm is applicable.
The thin film device mentioned above may be thin film transistor (TFT). Thus, high-performance TFT can be transferred (formed) arbitrarily onto a desired kind of secondary destination-of-transfer part. In this manner, a variety of electronic circuits can be mounted on the destination-of-transfer part.
The transfer process according to the present invention may be carried out repetitively on the secondary destination-of-transfer part which is larger than each substrate mentioned above. Thus, a plurality of object-of-transfer layers can be transferred to the secondary destination-of-transfer part.
A large-sized circuit panel comprising highly reliable thin film devices can be produced by carrying out thin film device pattern transference a plural number of times using a highly reliable substrate repetitively or using a plurality of substrates.
By carrying out the transfer process of the present invention repetitively on the secondary destination-of-transfer part which is larger than the abovementioned substrate, a plurality of object-of-transfer layers, each having different design rule levels, may be transferred to the secondary destination-of-transfer part in a single-sheet form.
For instance, where a plurality of circuits of different kinds (including functional blocks) are to be mounted on a single substrate, elements and wiring lines in these circuits may differ in size (i.e. so called design rule) according to the required characteristics. In such a case, by carrying out transference of each circuit using the transfer method of the present invention, a plurality of circuits, each having different design rule levels, can be formed on the secondary destination-of-transfer part in a single-sheet form.
Using the transfer method of the present invention, thin film devices or thin film integrated circuit devices may be configured on the secondary destination-of-transfer part through transfer processing. For example, a single-chip microcomputer comprising thin film transistor (TFT) can be mounted on a synthetic resin substrate.
According to the present invention, it is possible to form an active matrix board comprising a pixel portion which includes thin film transistors (TFTs) arranged in a matrix and pixel electrodes connected with respective ends of the thin film transistors, wherein the method defined in any one of claims 1 to 16 is used in transferring the thin film transistors of the pixel portion for fabrication of the active matrix board. In this case, using the transfer method of the present invention, the thin film transistors for the pixel portion are transferred to the secondary destination-of-transfer part to produce an active matrix board. Thus, since an arbitrary kind of substrate (secondary destination-of-transfer part) is selectable without consideration of restrictive manufacturing process conditions, a novel type of active matrix board may be realized.
According to the present invention, it is possible to produce an active matrix board which comprises thin film transistors serving as the pixel portion corresponding to a first design rule level and thin film transistors serving as driver circuits corresponding to a second design rule level. Both the pixel portion and driver circuits can thus be mounted on the active matrix board. However, the design rule levels of the pixel portion and driver circuits differ from each other. For example, a degree of circuit integration can be increased by forming a driver circuit thin film pattern using mono-crystal silicon transistor fabrication equipment, for example.
Through use of the technique mentioned above, a liquid crystal display device can be manufactured. For example, it is possible to realize a liquid crystal display comprising a plastic substrate, which can be curved flexibly.
Using the transfer method of the present invention, an electronic apparatus having thin film devices transferred to a secondary destination-of-transfer part may be produced. In this application of the present invention, as the secondary destination-of-transfer part, a casing part of the apparatus may be adapted so that the thin film devices will be transferred on at least either the inside or the outside of the casing part.
According to another aspect of the present invention, there is provided a thin film device transfer method, comprising:
a first step of forming a first separation layer on a substrate;
a second step of forming an object-of-transfer layer containing a thin film device on the first separation layer;
a third step of removing the substrate from the object-of-transfer layer using the first separation layer as a boundary; and
a fourth step of attaching a destination-of-transfer part to the bottom of the object-of-transfer layer,
whereby the object-of-transfer layer containing the thin film device is transferred to the destination-of-transfer part.
In the manner mentioned above, transference of the object-of-transfer layer can accomplished using the separation layer and the destination-of-transfer part instead of combined use of first and second separation layers and primary and secondary destination-of-transfer parts. This method is practicable if the object-of-transfer layer itself has a shape retaining property. Because, under the condition where the object-of-transfer layer is capable of retaining a shape thereof, it is not required to support the object-of-transfer layer with the primary destination-of-transfer part. In this case, the object-of-transfer layer may be provided with a reinforcing layer as well as a thin film device layer. dr
FIG. 1 is a sectional view showing a first step in a first embodiment of a thin film device transfer method according to the present invention;
FIG. 2 is a sectional view showing a second step in the first embodiment of the thin film device transfer method according to the present invention;
FIG. 3 is a sectional view showing a third step in the first embodiment of the thin film device transfer method according to the present invention;
FIG. 4 is a sectional view showing a fourth step of the first embodiment of the thin film device transfer method according to the present invention;
FIG. 5 is a sectional view showing a fifth step in the first embodiment of the thin film device transfer method according to the present invention;
FIG. 6 is a sectional view showing a sixth step in the first embodiment of the thin film device transfer method according to the present invention;
FIG. 7 is a sectional view showing a seventh step in the first embodiment of the thin film device transfer method according to the present invention;
FIG. 8 is a sectional view showing an eighth step in the first embodiment of the thin film device transfer method according to the present invention;
FIG. 9 is a sectional view showing a ninth step in the first embodiment of the thin film device transfer method according to the present invention;
FIG. 10 is a graph showing variations in transmittance of laser wavelength to a first substrate (substrate 100 indicated in FIG. 1);
FIG. 11 is a sectional view showing a first step in a second embodiment of the thin film device transfer method according to the present invention;
FIG. 12 is a sectional view showing a second step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 13 is a sectional view showing a third step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 14 is a sectional view showing a fourth step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 15 is a sectional view showing a fifth step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 16 is a sectional view showing a sixth step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 17 is a sectional view showing a seventh step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 18 is a sectional view showing an eighth step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 19 is a sectional view showing a ninth step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 20 is a sectional view showing a tenth step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 21 is a sectional view showing an eleventh step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 22 is a sectional view showing a twelfth step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 23 is a sectional view showing a thirteenth step in the second embodiment of the thin film device transfer method according to the present invention;
FIG. 24 is a sectional view showing a fourteenth step in the second embodiment of the thin film device transfer method according to the present invention;
FIGS. 25A and 25B are perspective views showing a microcomputer fabricated in a third embodiment of the present invention;
FIG. 26 is an explanatory diagram showing a structure of a liquid crystal display device in a fourth embodiment of the present invention;
FIG. 27 is a sectional structure diagram showing main part of the liquid crystal display indicated in FIG. 26;
FIG. 28 is a circuit scheme for explanation of main part of the liquid crystal display indicated in FIG. 26;
FIG. 29 is a sectional device diagram showing a first step in a method of fabricating an active matrix board according to the present invention;
FIG. 30 is a sectional device diagram showing a second step in the method of fabricating the active matrix board according to the present invention;
FIG. 31 is a sectional device diagram showing a third step in the method of fabricating the active matrix board according to the present invention;
FIG. 32 is a sectional device diagram showing a fourth step in the method of fabricating the active matrix board according to the present invention;
FIG. 33 is a sectional device diagram showing a fifth step in the method of fabricating the active matrix board according to the present invention;
FIG. 34 is a sectional device diagram showing a fifth step in the method of fabricating the active matrix board according to the present invention;
FIG. 35 is an explanatory diagram showing a fifth embodiment of the thin film device transfer method according to the present invention;
FIG. 36 is an explanatory diagram showing a sixth embodiment of the thin film device transfer method according to the present invention;
FIG. 37 is an explanatory diagram showing a first light irradiation step in a seventh embodiment of the thin film device transfer method according to the present invention; and
FIG. 38 is an explanatory diagram showing a second light irradiation step in the seventh embodiment of the thin film device transfer method according to the present invention.